Low data volume satellite communication system

ABSTRACT

Systems are disclosed for a communication system optimized for low data volume communications. In embodiments of the invention, a terminal in the communication system is configured to communicate with a satellite and a terrestrial hub using the same communications architecture, such as an air interface. In embodiments of the invention, the terminal sends a burst comprising a message to network infrastructure at a pre-scheduled time such that the network infrastructure can derive a terminal identity for the terminal by the time of the burst without having to include terminal identity information in the message. The terminal can communicate with the network infrastructure either through the satellite or through the terrestrial hub.

BACKGROUND

The present invention relates generally to the field of satellitecommunication systems. More specifically, the present invention relatesto embodiments of satellite communication systems suited to low datavolume communications.

A conventional Mobile Satellite System (MSS) can be configured toprovide services, such as voice and packet data communication throughoutthe world. Referring now to FIG. 1, a typical MSS 100 comprises one ormore geostationary satellites 102, one or more Gateway Stations (GS)104, and one or more Satellite Terminals (ST) 106. The STs 106 caninclude mobile terminals (handsets), vehicle terminals, and/or fixedterminals. The GS 104 can be configured with external interfaces toexisting fixed telecommunication infrastructure as well as to thewireless telecommunication infrastructure. For example, a GS 104 mayinterface to a Public Switched Telephone Network (PSTN) 108. Thesubsystems in Gateways can be oriented to various types of transmissionfunctionality, e.g., circuit-switched or packet-switched. The names ofthe subsystems vary between implementations. The term for all theground-based subsystems is Network Infrastructure 110, which includesthe GS and PSTN subsystems in FIG. 1. The satellite directs energy inthe forward link to areas on the ground called beams 112. The sameconcept of beam-forming is applied in the return link to separatelycapture the signals from terminals in each beam at the satellite.

Information is communicated in finite duration transmissions calledbursts. Bursts are composed of: waveforms related to physical layerfunctions such as detection and synchronization (e.g., pilot signals);and waveforms that contain modulated data. The modulated data includespayload fields and error detection fields (e.g., CRC). The payloadfields may contain control information (such as terminal identity), andapplication-related information. Any payload information that is notapplication-related is defined as an overhead.

Information can be transmitted via these satellites 102 using a CommonAir Interface (CAI). Existing satellite CAIs typically concentrate onefficient operation for relatively large quantities of data. Forexample, a voice call lasting one minute might involve 30 kB(kilo-Bytes) or more of information transmission in each direction.Packet data operations often involve even larger quantities of data,frequently in the MB (Mega-Byte) range. Providing a connection in aconventional MSS typically involves a sequence of steps including:

-   -   Requesting and establishing a link between a ST and network        infrastructure via a satellite;    -   Exchanging information characterizing the capabilities of the        end points;    -   Exchanging information describing the objectives and        configuration of the connection;    -   Transmitting the data and related acknowledgements; and    -   Exchanging information to terminate the connection.

Prior to transferring information, an ST 106 typically must “register”with the network. In addition, the ST 106 typically must “re-register”when it moves from one satellite beam to another. When an ST isregistered, the network infrastructure is aware that the ST is present,and the beam within which that ST can be located. After a ST isregistered, data exchanges can proceed. A conventional MSS data exchangemay start with establishing a communication channel. This may includesending a Random Access Channel (RACH) burst from a ST 106 to asatellite 102, which passes the RACH burst to a gateway 104. The RACHburst might include source information, such as a called party, terminalidentity (ID), terminal capabilities, the message intent (such asestablishing a packet connection) and possibly location information.Next an Access Grant Channel (AGCH) burst may be sent from the gateway104 to the ST 106 to establish a bidirectional traffic channel forfurther exchange of information. A typical AGCH burst can provide otherinformation, such as an indication of available resources for the ST106. Security information may be exchanged back and forth between thegateway 104 and ST 106. Further capability information, such as maximumdata rate, may also be exchanged between the gateway 104 and ST 106.

After a communication channel is established, data may be sent betweenthe ST 106, satellite 102, and gateway 104. The data may be sent inmultiple messages. Each message includes header and protocol overhead,which will vary in quantity depending on the scenario, and can amount toapproximately 20% of the message. Acknowledgement messages (ACK) arealso sent to acknowledge the successful receipt of the data messages.If, the data messages are not successfully received, aNon-Acknowledgement message (NACK) is sent and the data message(s) areresent.

After data communications are complete, the ST 106 will send a “done”message to the satellite 102 which gets passed on to the gateway 104and, if the “done” message is successfully received, a terminationmessage is sent to the ST 106 acknowledging receipt of the “done”message. For large quantities of data, the exchanges other than“transmitting the data”, can correspond to a reasonable overhead.However, for smaller data exchanges, the overhead can significantlyimpact efficiency.

SUMMARY OF THE INVENTION

One embodiment of the invention relates to a communication system,comprising at least one terminal in wireless communication with networkinfrastructure, wherein the at least one terminal is configured tocommunicate with the network infrastructure by sending a burstcomprising a message at a pre-scheduled time such that the networkinfrastructure can derive a terminal identity for the at least oneterminal by the time of reception of the burst without having to includeterminal identity information in the message. The pre-scheduled time forsending a burst can be part of pattern of pre-scheduled times at whichthe at least one terminal sends bursts, with the pre-scheduled times inthe pattern being set a fixed period from each other. The networkinfrastructure can consists of at least one network hub and a centralserver.

The time-framing of an ST-transmitted burst can be derived from waveformpatterns that are associated with the payload of the burst. Thetime-framing of transmission from the network infrastructure can bederived from waveform patterns that are associated with the BroadcastControl Channel (BCCH) bursts.

The communication system may also comprise at least one terrestrial hubin wireless communication with the at least one terminal and centralserver to provide an alternative communication path between thesatellite terminal and central server. In this case, the satelliteterminal receives messages from the satellite but sends the bursts tothe terrestrial hub which passes information from the bursts on to thecentral server.

At least one satellite relay may also be included in the satellitecommunication system. The satellite relay communicates with thesatellite and the terminal and provides an alternative communicationpath between the terminal and the satellite for situations in which theterminal may be shadowed from the satellite. The satellite relay can beconfigured to receive a burst from a terminal, apply a frequency offsetto the burst, and then forward the frequency offset burst to thesatellite. The satellite relay can also be configured to receive amessage from the satellite, apply a frequency offset to the message, andforward the frequency offset message to the terminal.

The network infrastructure can be configured to determine the success orfailure in reception of a burst from a satellite terminal and, inresponse, send an acknowledgement message back to satellite terminal.The acknowledgement message may consist of either an ACK indicatingsuccessful reception of the burst, or a NACK indicating failure ofreception of the burst. The energy required to transmit theacknowledgement message can be set to be different depending on whetherthe acknowledgement is an ACK or a NACK. For example, the energyrequired to transmit the acknowledgement message can be zero (ornegligible) if the acknowledgement message is an ACK.

The network infrastructure can also be configured to send broadcastsystem information to every terminal in communication with thesatellite. The broadcast system information may comprise a variety ofinformation including an update to the pre-scheduled time that theterminal sends the burst, an indication describing whether or not acommunications channel is shared with other services, networkinformation describing multiple networks in which the terminal maycommunicate, or control information including a location in time ofhigher-power frames. The update to the pre-scheduled time that theterminal sends the burst may be sent in response to unexpectedsituations in which the satellite is operating near capacity. If that isthe case, the broadcast system information may include parameters fordetermining a delay to be applied to the pre-scheduled time to avoid theunexpected capacity issues being experienced by the satellite.

Another embodiment of the invention relates to a method forcommunicating in a communication system having at least one satelliteterminal in wireless communication with network infrastructure, themethod comprising the at least one terminal waking up at a pre-scheduledtime, the at least one terminal acquiring and synchronizing to a forwardlink channel of the communication system, the at least one terminalsending a burst comprising a complete message at a pre-scheduled timesuch that the network infrastructure can derive a terminal identity forthe at least one terminal based on the time of reception of the burstwithout having to include terminal identity information in the message,and the at least one terminal going back to sleep after the burst isreceived by the network infrastructure. Time-framing of the burst can bederived from information supplied in the payload of the burst. Theterminal may be configured to repeating the waking up, acquiring andsynchronizing, and sending steps based on a pattern of pre-scheduledtimes a fixed period from each other.

Further features of the present invention, its nature and variousadvantages will be more apparent from the accompanying drawings and thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a conventional Mobile Satellite System(MSS).

FIG. 2 is a schematic diagram of one embodiment of a satellitecommunication system according to the present invention.

FIG. 3 is a schematic diagram of another embodiment of a satellitecommunication system according to the present invention showing multiplesatellites in the communication system.

FIG. 4 is a schematic diagram of a satellite communication systemaccording to the present invention showing satellite beams formed on theEarth's surface.

FIG. 5 is schematic diagram of another embodiment of a satellitecommunication system according to the present invention showing aterrestrial hub in the satellite communication system.

FIG. 6 is a schematic diagram of another embodiment of a satellitecommunication system according to the present invention showing analternative communications arrangement with a terrestrial hub.

FIG. 7 is a schematic diagram of another embodiment of a satellitecommunication system according to the present invention showing asatellite relay in the satellite communication system.

FIG. 8 is a schematic diagram showing signaling elements contained in aburst involved in a scheduled transmission according to one embodimentof the subject invention.

FIG. 9 is a schematic diagram showing modulation and acknowledgement andpower setting information for scheduled transmissions according to oneembodiment of the subject invention.

FIG. 10 is a schematic diagram showing the payload content of a BCCHburst according to one embodiment of the subject application.

FIG. 11 is a flow chart illustrating an exemplary quietening process interms of frame numbers according to one embodiment of the subjectinvention.

FIG. 12 is a schematic diagram showing the protocol associated with asuccessful scheduled transmission according to one embodiment of thesubject invention.

FIG. 13 is a schematic diagram showing several terminals in a beam whereeach terminal executes scheduled transmissions according to oneembodiment of the subject invention.

FIG. 14 is a flow chart illustrating an exemplary scheduled transmissionas seen by a terminal.

FIG. 15 is a schematic diagram showing the timing of burststransmissions from multiple terminals with scheduled transmissionsaccording to one embodiment of the subject invention.

FIG. 16 is a flow chart illustrating a terminal reading systeminformation according to one embodiment of the subject invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described in detailwith reference to the drawings, which are provided as illustrativeexamples so as to enable those skilled in the art to practice theinvention. Notably, the figures and examples below are not meant tolimit the scope of the present invention to a single embodiment, butother embodiments are possible by way of interchange of or combinationwith some or all of the described or illustrated elements. Whereverconvenient, the same reference numbers will be used throughout thedrawings to refer to same or like parts.

Where certain elements of these embodiments can be partially or fullyimplemented using known components, only those portions of such knowncomponents that are necessary for an understanding of the presentinvention will be described, and detailed descriptions of other portionsof such known components will be omitted so as not to obscure theinvention. In the present specification, an embodiment showing asingular component should not be considered limiting; rather, theinvention is intended to encompass other embodiments including aplurality of the same component, and vice-versa, unless explicitlystated otherwise herein. Further, the present invention encompassespresent and future known equivalents to the components referred toherein by way of illustration.

Embodiments of the subject invention can be configured to communicatesmall quantities of data much more efficiently than a conventional MSS.In one embodiment, information transmitted between terminals and networkinfrastructure is sent in bursts that are intended to communicate acomplete message without all the overhead used in establishing andterminating a connection associated with convention satellitecommunication systems. The bursts are sent in predetermined formats, atpredetermined times so that the identity of the terminal can be easilydetermined eliminating the need for much of the overhead of aconventional MSS.

As shown in FIG. 2, an exemplary low data volume satellite communicationsystem 200 can comprise at least one satellite 202, at least onesatellite terminal 208, and network infrastructure which can include atleast one ground-based satellite hub 206, and a central server 204.Communication signals can be passed in both directions between thesatellite 202 and satellite terminal 208 with uplink signals being sentfrom the satellite terminal 208 to the satellite 202 and downlinksignals being sent from the satellite 202 to the satellite terminal 208.In fact, many satellite terminals 208 may be connected via eachsatellite link. The satellite 202 can also be configured to passcommunications signals to and from the ground-based satellite hub 206and the satellite hub 206 can be configured to pass data to and from thecentral server 204.

Satellite terminals 208 may comprise communications devices capable oftransmitting information to, and receiving information from, networkinfrastructure via a satellite 202. Satellite terminals 208 may also beconfigured to communicate directly with network infrastructure as shownin FIG. 6, such as the ground-based terrestrial hub 610, using the samechannels. Downlink signals can be sent to a satellite terminal 208 usinga forward link 212 and uplink signals transmitted from the satelliteterminal 208 can be transmitted using a return link 210.

A satellite 202 can provide services across a set of channels within anarea on the surface of the earth, or above it, called a beam. Forexample, within each beam, a forward link 212 Broadcast Control Channel(BCCH) can provide system information to satellite terminals 208. Apilot channel can provide a known waveform that enables detection ofwaveforms and a reference for demodulation of other bearers. A separatepaging channel (PCH) can be used to transmit requests for connectivityto satellite terminal 208. ACK and power control channels can also betransmitted in the forward link 212 in response to bursts sent fromsatellite terminals 208. Traffic channels (TCH) can be used in both theforward 212 and return 210 links to convey payload information. A RACHcan be used by the satellite terminals 208 to request establishment of aconnection. Actual communication links can operate at different datarates. The lowest rate (which provides the highest link margin) can besupported by the most robust (or nominal) burst format.

Support for multiple networks, such as multiple satellite operators, andsupport for evolution to future networks can be included in embodimentsof the invention. In one embodiment, this support can be implementedusing the broadcast system information. The system information can beused to convey network information. As such, a terminal 208 may receivethe information needed to operate in a different or new network via thesystem information. The system information can also be used to providephysical layer flexibility in a satellite terminal 208 viasoftware-defined radio features in the satellite terminal 208. Supportfor multiple data rates can also be provided. In one embodiment,different transfer rates can be configured by using Walsh codes tochange the length of sequences used in information spreading, while alldata rates share a common transmission structure so they can operatesimultaneously.

FIG. 3 illustrates another embodiment of the invention showing a network300 with multiple satellites 302 and 304. Satellite 302 is configured tocommunicate with multiple satellite hubs 306, 308 and multiple satelliteterminals 310, 312, 314. The satellite hubs 306, 308 are also configuredto communicate with a central server 316. The central server 316 is alsoconfigured to communicate with satellite hub 318, which is connected tosatellite 304. Satellite 304 is also connected to satellite terminal320. As can be seen, the embodiment shown in FIG. 3 consists of anetwork 300 having several hubs 306, 308, 318 and several satellites302, 304.

The communication links between satellite terminals and a satellite, forexample, satellite terminals 310, 312, 314 and satellite 302, can usemultiple sets of resources which can be characterized by a set ofparameters. For example, the parameters could define carrier frequenciesof all usable channels, chip rates, channel filtering, etc. When a fixedgroup of parameter settings is used in communication with a satellite,the associated link is termed a space relay. It is possible for multiplespace relays (with different parameter settings) to operate over asingle satellite. A set of space relays using the same parametersettings is usually termed a network. A system may contain multiplenetworks, where those networks could use different parameter settings.

As mentioned briefly above, embodiments of the invention can beconfigured to communicate small quantities of data much more efficientlythan a conventional MSS by transmitting information in prescheduledbursts that are intended to communicate a complete message without allthe overhead used in establishing and terminating a connectionassociated with conventional satellite communications systems. The bursttransmission configuration of embodiments of the subject invention,which is described in more detail below, provides enhanced transmissionefficiency for low data volume communications in a satellitecommunication system. This scheduled transmissions approach can beparticularly useful in applications with regular reporting by satelliteterminals established on a long-term basis, such as utility metering inwhich the satellite terminals monitor and report consumer utility usage.

In some embodiments of the invention, satellite terminal identities arenot transmitted but are derived at the receiver based on the time of theburst arrival. The payload portions of bursts can be used to derivetime-framing, reducing overhead such as specific synchronizationchannels. It is also possible to avoid the exchange of capabilityinformation by mapping capabilities to the satellite terminal identity.Various embodiments can include efficient rescheduling of groups oftimed transmissions to react to busy hour changes. The rescheduling oftransmissions can be done based on pre-arranged alternative scheduleswhich can be controlled via the broadcast system information. Also,power control suited to a low duty cycle and low overhead operation canbe used. If GPS signals are being tracked, framing can be synchronizedto GPS time enabling quicker synchronization. Enhanced margin operatingoptions can also be included. For example, high priority communications,such as emergency calls, can be configured for transmit-only terminalswhich are not configured to receive satellite signals. Alternatively,paging to request a special format burst transmission can be configuredwhere the special format trades the quantity of information within theburst for higher probability of detection. Higher forward link power mayalso be provided in pre-defined patterns with a low duty cycle thusenabling link-constrained terminals to receive forward link bearers at alow rate. Various additional features of embodiments of the inventioncan include acknowledgements with low average power based on zero powerACKs as described in more detail with reference to FIG. 9 and/orterrestrial expansion of service as described in more detail below.Terrestrial expansion can include enabling satellite terminals toreceive transmission from satellites but to transmit to local receiverson the ground enabling higher throughput for scheduled reporting, etc.Alternatively, or in addition, hub equipment can be configured toperform both transmission and reception. Frequency shifting relays canalso be used. These relays can be primarily aimed at reaching heavilyshadowed terminals such as terminals with an obstructed view of thesatellite.

One exemplary implementation of an embodiment of a burst message whichuses a robust format is described herein (in terms of the number ofinformation bits, etc.) using a particular network configuration. Thisdescription of such an exemplary implementation addresses the modulationof payload information. Other elements of transmitted waveforms caninclude: pilot, acknowledgement, and power setting communications. Forthe purposes of this explanation, bursts can be described as transmittedwaveforms communicating information.

In one embodiment, a burst can be formed as follows:

-   -   Input contains 112 payload information bits;    -   An appended 16-bit Cyclic Redundancy Code (CRC) yielding 128        uncoded bits;    -   Error correction coding, at rate 1/4, yielding 512 coded bits;    -   Each coded bit can be spread using a 256 bit Walsh code,        yielding a total burst length of 2¹⁷ bits;    -   A 1024 bit Gold code can be combined (in this case via an XOR        function) with groups of 4 coded bits (each spread by a 256        Walsh code), wherein each such group has the duration of a        timeslot. Each quarter of a timeslot, associated with a Walsh        code, is called a symbol. That is, each coded bit corresponds to        a symoble. There can be 128 timeslots and 512 symbols in each        burst;    -   Each of the 2¹⁷ bits in a burst can be transmitted as a chip        (i.e. a filtered waveform) with time spacing (from        chip-to-chip), of a chip period; and    -   The time taken to transmit 2¹⁷ chips is a frame.

The timing of bursts transmitted by terminals can be defined in terms ofreturn transmit slots, which are times within frames that are identifiedby return transmit slot indices. The times are selected to provide goodperformance in reception of the bursts. More specifically, the definedtimes reduce the probability of simultaneous reception of bursts fromdifferent terminals that are aligned in timeslots.

In one embodiment, an exemplary network configuration can include thefollowing parameters:

-   -   Forward Link Carrier Frequencies and numbering (Absolute Radio        Frequency Channel Number or “ARFCN”);    -   Chip Rate;    -   Filter characteristics, e.g. Roll-Off factor of Square-Root        Raised Cosine; and    -   Frame Reference to enable time definition.

Sample parameter values in one exemplary embodiment can be set asfollows:

-   -   Forward carrier frequencies at 1,525,000,000+31,250*N; where        1≤N≤1,087    -   Chip Rate=23,400 cps    -   Roll-Off Factor=0.35    -   Frame Reference based on GPS time, starting at a particular date        and time, e.g.

UTC (midnight) of Jan. 1 to 2 2017.

The 512 coded bits transmitted can each be associated with a 256-bitWalsh coded sequence. Each Walsh coded sequence can be selected from oneof 256 possibilities, each defined by a Walsh Code Index. Selection ofthe Walsh codes can give a degree of freedom in the design of a system.For example, the set of Walsh codes used for transmission of BCCHchannels can be a key to Forward Link synchronization. As mentionedabove, conventional satellite communications systems typically do nothave air interface capabilities of the type described herein withrespect to various embodiments of the invention, which are capable ofcommunicating a complete message without establishing a connectioninvolving several or many burst transmissions.

Scheduled transmissions can be a key capability in various embodimentsof the invention. The following description outlines various embodimentsof scheduled transmission establishment and execution.

During terminal registration, the central server could establishscheduled transmissions to be performed by that terminal. The followingset of parameters provides an example of the information that may passedto the terminal. In one embodiment, 54 bits of transmitted informationconsist of 3 parameters and the combination of these parameters can becalled an Information Element. For example, Information ElementTI_IE_Sched_TX_Config can comprise parametersTI_ELMT_Sched_TX_First_Frame, TI_ELMT_Sched_TX_Frame_Incr, andTI_ELMT_Sched_TX_Timeslot. TI_ELMT_Sched_TX_First_Frame can comprise 23bits, TI_ELMT_Sched_TX_Frame_Incr can comprise 23 bits, andTI_ELMT_Sched_TX_Timeslot can comprise 8 bits. The first element,TI_ELMT_Sched_TX_First_Frame, can define the 23. Least Significant Bits(LSBs) of the frame number of the first transmission (e.g., it can havea range of ˜543 days if the chip rate is 23.4 kcps). The second element,TI_ELMT_Sched_TX_Frame_Incr, can define the number of frames betweentransmissions (which can also have a range of ˜543 days if the chip rateis 23.4 kcps). The third element, TI_ELMT_Sched_TX_Timeslot, can defineon which of the timeslots within the selected frame the terminal shouldbegin transmission.

The assignments of frames and timeslots can be arranged to ensure thateach terminal has a unique transmission start time. For example, whenthe time of the first scheduled transmission is approaching (perhaps 100seconds beforehand), the terminal can acquire and then receive theforward link control channel, the BCCH. The terminal can then read someof the content of the System Information to determine whether it shouldproceed with the scheduled transmission. That is, the terminal can checkthat channel quietening is not active and that transmission is enabled.The terminal may then execute a protocol that begins with transmissionof the scheduled transmission burst 1202, as shown in FIG. 12. As thetransmission time approaches (perhaps a few seconds beforehand), theterminal can prepare the burst for transmission and the associatedreal-time control registers can be programmed to enable the transmissionto begin. Then, at the selected transmission time, the burst can betransmitted. As shown in FIG. 12, the terminal will then receive a burst1204 from the hub. If that burst contains an ACK indication, then theprotocol will end, and the terminal will return to a dormant state andwait for the next scheduled transmission. If a NACK indication isreceived, then the terminal will retransmit the burst at a pre-definedretransmission time. Retransmissions will continue until an allowedmaximum number of retries is reached.

FIG. 13 illustrates one embodiment of the subject invention in whichmultiple terminals, located in the same beam, execute scheduledtransmissions. Terminals located in beams other than the one illustratedhere may use different Gold sequences and thus are typically not seen byHub 1312. The four terminals 1304, 1306, 1308, 1310 can be assigneddifferent transmit times as illustrated in FIG. 15. TheTI_IE_Sched_TX_Config Information Element can be used to assign thedifferent scheduled transmission times for bursts 1502, 1504, 1506, and1508. As shown in FIG. 13, terminal 1304 can be a mobile terminalattached to a moving vehicle, terminal 1306 can be a mobile terminalattached to a house pet, terminal 1308 can be a mobile terminal attachedto a bicycle, and terminal 1310 can be a mobile terminal attached to aboat. The terminals 1304, 1306, 1308, 1310 can be configured to sendburst messages to the network infrastructure (satellite hub 1312 andcentral server 1314) through the satellite 1302 at differentpre-determined times such that the network infrastructure can match upthe burst messages with the sending terminal based on the time ofreception of the burst message at the network infrastructure. In thisway, message overhead can be reduced because terminal identityinformation as well as other overhead typically found in a conventionalcommunication system, is not needed.

FIG. 14 illustrates an exemplary scheduled transmission process for aterminal according to one embodiment of the invention. Each terminal ina system can implement scheduled transmissions as shown in FIG. 14.According to FIG. 14, the terminal first wakes up 1402, then acquiresthe pilot waveform 1404 resulting in resolution of timeslot boundaries.Burst framing can be determined by the terminal based on Walsh codesequences associated with the BCCH 1406. The terminal then reads theSystem Information 1408 and, based on the content of the SystemInformation, the terminal can determine whether to proceed 1410 with thescheduled transmission, or to reschedule for a later time 1412. If it isdetermined that the transmission should proceed, the terminal will waitfor the assigned time 1414 and then transmit the burst 1416 at thescheduled time. After transmission, the terminal will receive anacknowledgement (ACK or NACK) 1418. In the event an ACK is received, thescheduled transmission will be completed 1424 and the terminal will goback to sleep. If a NACK is received, the terminal will prepare toretransmit the burst 1422, and then repeat the transmission andacknowledgement steps 1414, 1416, 1418, 1420 until the burst issuccessfully transmitted.

The maximum number of retransmission attempts can be limited by aparameter that can be delivered via System Information. The hub receivercan be configured to receive transmitted bursts from each of the fourterminals. The spreading codes (Gold and Walsh) can be used to enablethe separate reception of each burst. These bursts may overlap in time.Low auto-correlation of the spreading sequences can enable reception ofburst which overlap in time. The time at which the bursts are receiveddepends on the transmission time and the length of the signal paths. Themaximum difference in signal delay across a beam is usually less thanthe difference between the assigned transmission times. Signal delay canarise both in the forward link, from which the terminal derives its timereference, and the return link, through which the transmission passes.As a result, the arrival time of each burst can be unambiguously mappedto the source terminal.

As shown in FIG. 4, in an exemplary satellite communication system 400,a satellite 402 may provide communication links to areas on the Earth'ssurface. Using a beam former, the satellite 402 can direct signals 404to areas on the ground, for example, creating beams 408, 410, 412.Typical satellites create hundreds of beams, where each beam is hundredsof kilometers in diameter. Within a beam, multiple carrier frequenciesmay provide connectivity (in each direction 404, 406). Neighboring beams(such as 408 and 410) can use the same carrier frequency. A satelliteterminal 414 within a beam 412 can be assigned resources associated withthat beam 412. Multiple satellites may create beams that cover asatellite terminal, which means that operational mapping between eachsatellite terminal and a satellite (and beam) should be determined.

In some instances, particularly when the number of satellite terminalswithin a beam is large, it may be advantageous to provide a terrestrialreceiver for the satellite terminal transmissions. FIG. 5 illustrates anembodiment of a satellite communication system 500 in which a satelliteterminal 506 receives downlink signals 504 from a satellite 502 buttransmits 508 to a terrestrial hub 510. This architecture can beparticularly advantageous to applications, such as utility metering, inwhich most of the communications traffic is in the direction fromsatellite terminals 506 to a central server 512. In this embodiment,information from satellite terminals, such as 506, within wirelesscommunications range of the terrestrial hub 510 could be received by theterrestrial hub 510. Terrestrial reception could use the same terminaltransmit channel as that used for the satellite link. The terrestrialhub 510 could be configured to be aware of the delay to the satellite502, and thus could determine the expected time and frequency ofreceived bursts. As such, the satellite terminal 506 need not be awareof terrestrial reception.

The terrestrial hub 510 can be configured to change the satelliteterminal 506 transmit power to match the needs of terrestrial reception.In many cases, this can result in significant power reduction. The powerchange can be implemented in a number of different ways. For example,the power change can be a gradual adjustment after each transmission orthe power can be adjusted via a paged exchange where the terrestrial hub510 individually instructs (via the central server 512, the satellitehub 514, and the satellite 502) each satellite terminal 506 to make achange in power. The range of power control of the satellite terminal506 to support satellite operation can be typically approximately 15 dB.For terrestrial operation, the required range can increase toapproximately 80 dB due to the variation in path loss in a terrestrialenvironment. The dynamic range can be reduced in a number of ways suchas using higher data rates when transmitting close to the terrestrialhub 510 or using multi-user detection at the terrestrial hub 510 toreduce the sensitivity to the difference in received power levelsbetween terminals 506.

In another embodiment, communications in both directions may be providedterrestrially, such as when the volume of traffic in both the uplink anddownlink directions becomes large. As shown in FIG. 6, the terrestrialhub 610 and satellite terminal 606 can be configured to communicate inboth the uplink 608 and downlink 604 directions terrestrially. Theterrestrial hub 610 and satellite terminal 606 can be configured to useadditional channels. As conventional satellite systems may use TimeDivision Multiple Access (TDMA) within each beam (and frequency reuseover several beams), the number of channels available for terrestrialCode Division Multiple Access operation with a beam could be a sizableportion of the satellite system's 600 available spectrum.

In general, a satellite must have line-of-sight access to a satelliteterminal in order to communicate with the satellite terminal. As shownin FIG. 7, a satellite terminal 704 may be shadowed from a satellite 702if the line of sight between them is obscured, such as by a tree 706 orother obstruction. Embodiments of the invention can include terrestrialrelays 708 which are ground-based transceivers that provide links 710,712 to a shadowed satellite terminal 704.

A terrestrial relay 708 can be configured to receive forward link 714communications in one or more channels from a satellite 702. Theterrestrial relay 708 can then retransmit the content to the shadowedsatellite terminal 704. Retransmission can occur at another frequencywithin the allocated forward link band, but typically not used directlyfrom the satellite 702 within the beam containing the terrestrial relay708. For example, the terrestrial relay 708 may receive forward link 714communications from the satellite 702 at a carrier frequency of Fdn andthen retransmit 710 the information to the shadowed satellite terminal704 at a carrier frequency of Fdn+ΔFdn.

Similarly, the terrestrial relay 708 can also be configured to transmitreturn link communications 716 to the satellite 702. The terrestrialrelay 708 can receive return link signals 712 from the shadowedsatellite terminal 704, frequency shift the received signals, andretransmit the frequency shifted 716 to the satellite 702. For example,the return link carrier frequency may be transmitted at a carrierfrequency of Fup+ΔFup from the shadowed satellite terminal 704 and theterrestrial relay 708 may convert the carrier signal to Fup forretransmission to the satellite 702.

The frequency offsets (ΔFdn, ΔFup) applied to both the forward link 714and return link 716 can be advertised in the broadcast systeminformation. In addition, the signals passing through the terrestrialrelay 708 in both the forward link 714 and return link 716 can also havea fixed delay. For example, the delay may be set at 0.1 ms (with atolerance of 5 μs). The shadowed satellite terminal 704 can be aware ofthe delay in the terrestrial relay 708 (as this is either a systemconstant, or a period defined in the broadcast system information) andcan also be aware that they are using a terrestrial relay 708 (as thefrequency of the forward link 710 coincides with a relay assignment).The shadowed satellite terminal 704 can adjust its transmit timing suchthat its transmissions arrive at the satellite 702 at an intended time(i.e., the central server 720 and satellite hub 718 need not be awarethat the shadowed satellite terminal 704 is operating via a terrestrialrelay 708). Alternatively, status messages from the shadowed satelliteterminal 704 may inform the central server 720 whether or not it isoperating via a terrestrial relay 708.

The terrestrial relay 708 does not need to modify the content of datapassing through it. The terrestrial relay's 708 primary function can becarrier frequency conversion. In addition, the shadowed satelliteterminal 704 may be configured with sufficient margin in its schedulingof events to reduce the delay between reception and transmission ofsignals. For example, with a fixed delay of 01. ms in each direction inthe terrestrial relay 708, the change in delay at the shadowed satelliteterminal 704 could be 0.2 ms.

As described above, in various embodiments of the subject application,satellite terminals can communicate with network infrastructure usingscheduled burst transmissions that are intended to communicate acomplete message without all the overhead used in establishing andterminating a connection associated with conventional satellitecommunication systems. FIG. 8 illustrates one exemplary scheduledtransmission communication 800. During scheduled transmissions, asatellite terminal transmits a burst 802, where the content of the burstcomprises a pilot 804 and traffic 806. The central server can respondwith a hub transmission 808, which can contain responses to all thesatellite terminals that have transmitted over the period of a frame.The hub transmission 808 can include pilot and traffic signals 810, 812,as well as acknowledgements 814 and power settings 816.

In various embodiments of the invention, the acknowledgement informationcan be modulated. During each frame, the number of satellite terminalscheduled transmissions will be less than 256 (i.e., corresponding tothe number of orthogonal Walsh codes). As such, in the correspondingforward link frame, fewer than 256 acknowledgements (and 256 powercontrol signals levels) can be transmitted. A single (normal) Walsh codecan be assigned for acknowledgements and another single Walsh code canbe assigned for power control levels. Note that there are 512 coded bitswhere each of those is modulated by a normal Walsh code within eachburst. There can be two distinct categories of Walsh code basedspreading sequences: Normal Walsh codes (i.e., of length 256 for themost robust case) used to modulate each coded bit; and Long Walsh Codes,used to generate orthogonal sequences over the whole burst (i.e., oflength 512) where each bit applies to a symbol. To transmit a binaryvalue over the duration of a burst (e.g., ACK/NACK) a specific LongWalsh Code can be assigned to each satellite terminal. The mapping fromLong Walsh Codes to satellite terminals can be based on the returntransmit slot index. The 512 possible Long Walsh Codes are more thansufficient to support the number of satellite terminals which is lessthan 256 in number.

As the target Frame Error Rate (FER) falls below 1%, the expected ratioof ACKs to NACKs is at least 99:1. To conserve satellite power, it wouldbe beneficial to minimize the total energy needed to transmit thecombination of ACKs and NACKs. Received signals can be passed through amatched filter, synchronized in time and frequency, and correlated withthe known spreading sequences. The result can be viewed as a basebandequivalent 2-dimensional vector called a correlation vector.

FIG. 9 illustrates exemplary acknowledgement 902, 904 and power setting906 correlation vectors without noise or other impairments. To achieve adesired error rate in the acknowledgements, the distance in correlatedvector space between the constellation point associated with an ACK andthat associated with a NACK should exceed a minimum length. In oneembodiment 902, which shows an example with equal amplitudes for ACK andNACK modulation, this distance can be 2×D-an. One of the key drivers ofthe error rate can be the distance between the points, not theirlocations. The location of these constellation points in correlationspace is a degree of freedom, allowing the vectors to be moved such thatthe total associated energy can be minimized. For example, theassociated power may be 99×P-ack+1×P-nack, where P-ack and P-nack areproportional to the square of the amplitude of each vector. As there aremany more ACKs than NACKs, average power can be reduced by reducingP-ack and increasing P-nack. Average power is minimized when P-ack isclose to zero. In the case illustrated (with P-ack set to zero) by 904,the NACK-related correlation vector has twice the amplitude D-an;meaning that P-nack is 4 times what it would be if P-ack=P-nack as shownby 902. The average power required to transmit acknowledgements for case904 is about 4% of that required for case 902, which equates to areduction in power of approximately 96%. To achieve this performance,the decision threshold between the ACK and NACK related constellationpoints should be known. As the acknowledgements are transmitted in thepresence of pilot signals, the receiver can be configured to calibratethe location of the decision threshold.

The error rate for the acknowledgement process is typicallysignificantly better than for data traffic. For example, theacknowledgement process error rate can be lower than 0.1%. At the sametime, the error rate for NACKs can be traded against that foracknowledgements ACKs. This trade can be driven by the relative costs ofa mistake. For example, a NACK received as an ACK can result in failureto communicate the message, while an ACK received as a NACK can resultin an unnecessary retransmission (which would typically have lessimpact). According to various embodiments of the invention, theacknowledgement process can be implemented using existing physicalbearers, without specific changes to the physical layer design toimprove efficiency.

Initialization of communication between satellite terminals and thecentral server can be accomplished in a number of different ways. Forexample, scheduled satellite terminal transmissions can be used where apredefined time is established at which the satellite terminal wouldtransmit a fixed length message. A typical embodiment of this type canbe used in many different applications, such as, for example, regularutility meter readings. Another exemplary embodiment can usealarm-driven satellite terminal communications. In these embodiments,some event at the satellite terminal can initiate an exchange ofinformation. For example, opening a door can trigger an alarm that wouldbegin an information exchange. In still another embodiment, pagedcommunications can be used in which the central server initiates anexchange of information. One sample application of this type ofembodiment could be used to change parameter settings in the satelliteterminal.

Each satellite terminal can have a unique associated identity, denotedby a Mobile Device Identity (MDI), which can be a 64-bit value. Duringoperation, a satellite terminal can also be assigned a Temporary MDI(TMDI), which can be shorter, such as a 24-bit value. Duringalarm-driven communications the related satellite terminal will identifyitself, and for paged communications the satellite terminal can beidentified by the central server. This identification can be provided bythe TMDI. During scheduled transmissions, the satellite terminal can beaware of the transmission time and the central server can be aware ofthe identity of the satellite terminal that is configured to transmit atthe scheduled time. The central server can identify the satelliteterminal without having to read information from the content of thetransmission. In other words, identity information need not be includedin the transmitted information. For example, in a burst containing 112payload bits, the application-related information can be increased by27% by avoiding the transmission of 24-bit identity information. Inaddition, each satellite terminal can be configured to apply uniqueciphering to its transmitted data, providing another means of confirmingthe identity of a source satellite terminal.

When a satellite terminal powers up, it can acquire and synchronize tothe forward link channels of a satellite. This can be accomplished in anumber of ways. For example, with typical acquisition algorithms, thesatellite terminal can detect the presence of a pilot channel, which canbe used to determine estimates of the carrier frequency and timeslottiming. The start time of each timeslot can be resolved, but thelocation within each frame and the frame number are typically notdetermined by the acquisition algorithm. The BCCH can then be observedso that the frame boundaries can be determined.

FIG. 10 illustrates one exemplary BCCH payload structure 1000 accordingto various embodiments of the invention. The BCCH can use multiple knownWalsh codes, transmitted in a known sequence, while modulating thepayload information. By searching for correlation with the expectedpattern of Walsh codes, framing of the forward link can be resolved. Assuch, a separate synchronization channel is not needed thus savingsatellite power. As shown in FIG. 10, each BCCH burst payload canconsist of 128 timeslots 1002, each of which can contain four Walshcodes 1004 associated with four coded BCCH bits 1006. The boundaries ofthe timeslots can be resolved as described above. The same Walsh codecan be used for the 4 symbols of each timeslot. Over the 128 timeslots,16 different Walsh codes can be used. The Walsh codes can be arranged ina pseudo-random pattern that can be selected with the goal of maximizingthe distance between the correct framing and any offset of that framingby an integer number of timeslots. After establishing framing, thecontent of the BCCH can be received and the frame numbering can be readfrom system information.

Different satellite terminals can have different capabilities in partbecause applications associated with each satellite terminal may havedifferent requirements. For example, a mobile terminal, such as one usedfor tracking a vehicle may have different capabilities and requirementsthan a stationary terminal, such as one used for utility metering. Theparameters of protocols and other terminal characteristics establishedby the central server can depend on awareness of these capabilities.Additional differences may arise as the system evolves and terminalswith newer capabilities are introduced. However, because thecapabilities associated with satellite terminals in embodiments of theinvention can be mapped to their identities (MDIs), there is no need toexchange information related to terminal capabilities. When transmittinginformation, the formatting of data may depend on the associatedapplication, such as information related to electric meter reading orvehicular asset tracking. However, in embodiments of the invention,there is no payload overhead for defining field sizes, locations, etc.because the formatting of scheduled transmissions (and others) can alsobe mapped to terminal MDIs.

According to various embodiments of the invention, satellite terminalscan be configured to support applications that transmit data atpre-defined times based on regularly-spaced intervals, but where theprecise time of the transmission is not critical such as utility meterreadings. Other services, such as voice links, provided by a satellitecan involve concentrations of throughput at specific times of day, suchas during times of heavy voice traffic. Typically, satellite terminaltransmissions will be scheduled to avoid predicted busy hours. However,in some cases, satellite capacity may approach its limits due tounforeseen events. Under such conditions, scheduled transmissions can bereassigned based on parameters delivered via system information whichcan be read by the satellite terminals prior to transmission. Thisreassignment process, called scheduled transmission quietening, enablesdelaying of selected scheduled transmissions for a period of time. Whenrescheduling, a defined timeframe can be cleared of scheduledtransmissions and transmissions can be rescheduled over the followinghours. After conditions change, the system can return to normaloperation. Scheduled transmission quietening can be used to efficientlyenable management of satellite resources without individually changingthe transmission schedules of every impacted satellite terminal.

In order to implement scheduled transmission quietening in embodimentsof the invention, three system information parameters can be used todefine the real-time communication status. A scheduled transmissionquietening active flag (SI_quiet_flag) indicates that scheduledtransmissions should not be transmitted. This flag can be set to stayactive for a specified amount of time such as 2 hours. Additionalparameters are used to define the configuration of quietening. Oneparameter (SI_ELMT_quiet_period) can be used to define a period of time(the quietening delay) that is equal to or longer than the period ofquietening, where the parameter's 4-bit value is in units of 512 frames.A second single bit parameter (SI_ELMT_quiet_spread) can be used todefine whether the retransmissions are spread over 4 quietening delayperiods or 8 quietening delay periods. The selection of which of the 4(or 8) quietening delay periods to use for the retransmission can bebased on the Least Significant Bits of the terminal temporary identity(TMDI), which is a number known to both the terminal and theinfrastructure.

FIG. 11 illustrates an exemplary quietening process 1100. An operatormay initiate a period of quietening that can be applied to scheduledtransmissions at 1102. In doing so, the operator sets the quieteningparameters which are entered into the System Information broadcast inthe BCCH at 1104. For example, a period of 1.5 hours can be selectedduring which scheduled transmissions should be disabled (e.g., 9:00PM-10:30 PM). In addition, the quietening delay can be set (viaSI_ELMT_quietperiod), such as for 2 hours, and the number of delay stepsafter quietened transmissions can be set (via SI_ELMT_quiet_spread),such as for 4 times. Next a satellite terminal will wake up at 1106. Thesatellite terminal then determines whether or not quietening iscurrently enabled by reading the System Information entry,SI_quiet_flag, at 1108. If quietening is not enabled, the satelliteterminal proceeds with its scheduled transmissions at 1110. Ifquietening is enabled and the satellite terminal has an assignment for ascheduled transmission during the quietening period, such as 10:05 PM,the satellite terminal can read the quietening parameters from thesystem information at 1112. The satellite terminal can then determinethe rescheduled transmission time, at 1114, based on the quieteningparameters. In order to do so, the satellite terminal can select anumber between 1 and the number of delay steps after quietenedtransmission parameter, which, in this example, is 4. A deterministicreference known to the central server, such as Least Significant Bits ofthe TMDI, can be used to selecting the number. For example, in thiscase, the selected number may have a value of 3 (i.e., between 1 and 4).The transmission delay can then be calculated by multiplying theselected number (plus 1 to include the quietened period) by thequietening delay. In this case, since the quietening delay is 2 hours,the calculated delay is determined to be 8 hours or until 6:05 AM. Aftercalculating the transmission delay, the satellite terminal goes back tosleep, at 1118, until the delay expires. At 1120, after the delayexpires, the satellite terminal wakes up and, at 1122, the rescheduledtransmission begins. It should be noted that the transmissions that havebeen delayed by the quietening process are transmitted with differentCRC masks thus enabling the central server to distinguish rescheduledtransmissions from regularly scheduled transmissions.

During scheduled transmissions, the central server can respond to eachtransmission with an acknowledgement and a power setting level. As therecan be a correlation between the purposes of these values (i.e., lowpower is more likely to lead to failed communications and a NACK), thevalues can be interpreted as a pair when deriving power changes. Forexample, the following table provides an exemplary correlation betweenacknowledgement and power setting level.

Power Nominal Acknowl- Control Action for Next Power Case edgement LevelTransmission (dB) Change (dB) 1 ACK −1 Lower power by P-am −0.5 2 NACK−1 Raise power by P-nm +1.0 3 ACK +1 Raise power by P-ap +1.5 4 NACK +1Raise pwer by P-np +2.0

The actual power level changes can be controlled via system informationor by terminal specific reconfiguration. A power control correctionsignal could be transmitted within a range relative to a pilot signal(e.g., if the pilot signal amplitude is +10 units, the power controlcorrection signal amplitude can vary in one dimension of a base-bandvector representation such as between −1 unit and +1 unit as shown by906 in FIG. 9). The power control correction signal may be transmittedwith a level anywhere in this range, indicating different requestedchanges to the satellite terminal's transmit power. The power changes inthe above-table could be scaled by the level of the power controlcorrection signal. That is, control scaling could be analog. Thisapproach is possible due to the presence of the pilot signal, whichenables calibration signal levels.

Error rates for acknowledgements should be lower than those for themessage payload. For power control, on the other hand, high error ratesare typically more tolerable, as (for example) any reception of a NACKcan cause an increase in subsequent transmit power levels, irrespectiveof the received Power Control level. In exemplary implementations, theenergy associated with transmitting a NACK might be 8 dB lower thantraffic energy, while that for Power Control might be 23 dB lower thantraffic energy.

In summary, the key features of a scheduled transmission (for the mostcommon scenario in which no errors occur) are:

-   -   A message with no overhead (i.e., all payload bits are        application-related) is transmitted,    -   No energy is used sending the ACK,    -   Power setting for future transmissions (if transmitted) is sent        at a low power level.

Assuming 1% FER, and that Power Setting levels are transmitted, theaverage power transmitted in the forward link responding to eachscheduled transmission could be ˜1.1% of that of a forward link trafficburst.

The link margin of the forward link can be increased by raising thepower transmitted at the satellite. At the same time, satellite power isa precious resource. By occasionally transmitting BCCH bursts at higherpower levels, a trade between satellite power and/or link margin, anddelay can be provided. As a satellite tends to be limited by the totalinstantaneous transmit power, it is typically advantageous to cycle theincrease in power from beam to beam. For example, by arranging beams ingroups of 16, the BCCH power could be increased in each beam for1-out-of-16 BCCH bursts. By identifying the pattern of higher power BCCHbursts to the satellite terminals via system information, thoseterminals can target their reception to the higher power bursts forcases when they note that their receivers are operating at or belowthreshold signal-to-noise ratio. This same capability can be applied tolower average satellite power while still maintaining nominal linkmargin.

In some circumstances, a satellite terminal may not be able to detectthe forward link signals, such as when the satellite is shadowed. If, atthe same time, a terminal user requests an emergency alarm, thesatellite terminal may be able to transmit a related emergency message.If GPS timing is available, the satellite terminal may use this as abasis for synchronizing its transmitter. If not, the terminal maytransmit with synchronization based on its local reference oscillator.In this case, consideration shall be given to the potential frequencyerror, and how it might impact neighboring channels or how it mightimpact compliance with any regulatory requirements. Emergencytransmissions may include information such as the identity of thesatellite terminal and its location. These transmissions may be repeatedat times defined for each terminal. The time between repetitions isknown to the central server, which may attempt to combine multipletransmissions to reduce the error rate and thus identify the satelliteterminal and its location.

In some circumstances, a satellite terminal's location may be ofinterest, under conditions in which the satellite terminal is unable tosuccessfully transmit. For example, after a valuable item with anattached satellite terminal has been stolen, operators at the centralserver may invoke an emergency page. The hub can be aware of the pagingreception time of a satellite terminal. Prior to reception, thesatellite terminal can wake up and attempt to acquire the forward linkchannels. During the time the satellite terminal is attemptingacquisition, the forward link signal levels may be raised to increasethe link margin for both acquisition and the page. Once the satelliteterminal receives the page, it can respond by transmitting a completeburst with known content at the maximum power level. This can befollowed by a separate burst containing the terminal's location. Theoperators may also suspend other traffic in the return link during thetime the satellite terminal is transmitting. This may increase theprobability that the satellite terminal can be contacted and willrespond.

Satellite terminals may switch between networks, which may be providedby different satellite operators, to provide, among other things,continuity of service in the event of a satellite failure. Support forthis flexibility may arise in the satellite terminal implementation,such as, for example, carrier frequency flexibility, and in the systeminformation, which may include definitions of existing networks as wellas parameters enabling future networks to be defined. In situations inwhich a satellite terminal is covered by multiple networks, the centralserver may direct satellite terminals to a specific network. The systeminformation may include information related to multiple networkoperation centers such as frequencies, chip rates, filtercharacteristics, etc. The satellite terminals may be configured withprioritized network preferences and/or the central server may beconfigured to redirect satellite terminals to different networks. As newnetworks come into existence, the system information may be updated todescribe these new networks.

System Information (SI) can be a set of parameter values that arebroadcast from a Hub to all terminals in a beam. In typical systems, SIcan be arranged in a number of classes, enabling efficient management oftransmission of the information. For example, information that changesrarely, such as descriptions of space relays, can be placed in a classwith other rarely changing information. The priority, and likelihoodthat parameters will change, can drive the duty cycle at which eachclass of SI is transmitted. Particular SI parameters may be repeated inevery BCCH burst, and some may not be repeated for many BCCH bursts.Some information may change at any time, and should be read before aterminal transmits, e.g., flags that can disable transmission. Aterminal that is about to transmit should read these flags, but may notbe required to learn of all the available space relays that areavailable. In another scenario, a newly registering terminal may gothrough the process of reading all the SI, including descriptions ofspace relays.

The System Information associated with scheduled transmission quieteningcan provide an exemplary case. In FIG. 11, a terminal reads quieteningparameters from the System Information 1106 and 1112. FIG. 16 shows thisprocess in more detail, where the quietening parameters are:

-   -   SI_quiet_flag; a 1-bit value in Class-1. System Information    -   SI_ELMT_quiet_period; a 4-bit value in Class-2 System        Information    -   SI_ELMT_quiet_spread; a 1-bit value in Class-2 System        Information

In preparation for a scheduled transmission, the terminal can read theClass-1 information 1602, which can be included in every BCCH burst.From the Class-1 information, the terminal can learn the value of theSI_quiet_flag (i.e., that quietening is requested). Given thisinformation, the terminal cab be aware (a) that there will likely be asignificant delay before the transmission occurs, and (b) that it shouldread Class-2 information to determine the parameter settings for thequietening. The terminal, therefore, can proceed with reception of theClass-2 information 1610 and with determination of the values ofSI_ELMT_quiet_period and SI_ELMT_quiet_spread 1612. Note that theassignment of these parameters to Class-2 can be enabled by theavailable time for reading the content. In general, parameters can beassigned to the highest class number that enables the desired relatedoperation.

Embodiments of the invention can also be configured with securityfeatures such as authentication of each satellite terminal by thecentral server during registration, authentication of the central serverby the satellite terminals during registration, and/or ciphering of datatransferred between the satellite terminals and the central server toname a few. In one embodiment, these security features can beimplemented using a set of non-public keys that are stored at eachsatellite terminal and the central server. Compromise of the secret keysstored in a particular terminal would only impact that satelliteterminal. In other words, the keys stored in a particular terminal donot provide information related to other satellite terminals. Means foridentification of anomalous behavior by potentially-comprised terminalsmay be applied. For example, transmission from distant locations bymultiple terminals with the same identity could be flagged as apotential security threat.

Unlike in conventional systems, the security related processing inembodiments of the invention occurs at the central server rather than atthe hub. This approach provides enhanced security as secret keys neednot be moved to hub facilities and the list of temporary and permanentsecret keys for all satellite terminals can be maintained in onelocation. As embodiments of a system according to the present inventioncan be implicitly aware of the identity of every satellite terminal andall communications can pass through a single point and can be associatedwith specific owners, the motivation for compromising the system isinherently less than conventional systems.

Transmissions by satellite terminals can be prevented by the centralserver via the system information. In this way, it is possible to managesystem resources, such as power and bandwidth, when a space relayapproaches capacity. These controls typically apply at the time they areread by the satellite terminals. In other words, a satellite terminalmust read the system information within a specified period prior totransmission. Individual transmissions may be prevented based on avariety of things such as terminal class and/or communication mode.

In various embodiments of the invention, the maximum length of a mostrobust burst can be 1,024*128 (2¹⁷) chips, corresponding toapproximately 5.6 seconds at 23.4 kcps, with an information data rate ofapproximately 20 bps. By using shorter Walsh codes (in factors of 2steps), higher information data rates can be achieved. For example,rates of 40 bps, 80 bps, 160 bps, 320 bps, etc. can be achieved. Thelength of bursts can correspondingly be reduced with the informationcontent of each burst remaining fixed. Bursts can be structured tosupport different data rates. For example, forward broadcast controlchannels can operate at the most robust data rate. For forward trafficchannels and return link channels, bursts, configured during connectionestablishment can be configured for a factor of 2^(N) increase in datarate. For example, the number of timeslots in the burst can be reducedto 128/2^(N), the number of chips in the Walsh codes can be reduced to256/2^(N), the length and content of Gold codes can remain unchanged at1024 chips (a timeslot), and the number of coded bits per timeslot canincrease to 4×2^(N). Frame timing can be divided into sub-frames oflength 2^((17-N)). The hub can advertise the data rates of support forRACH, enabling the satellite terminals to select a supported rate. TheAGCH can use the same rate as the RACH, but may assign another rate forsubsequent communications.

In some situations, satellite terminals in an embodiment of theinvention may be aware of Global Navigation Satellite Systems (GNSS).These systems may provide reference timing signals that could be appliedas a reference for framing, etc. If satellite terminals are aware thatframing is based on GPS timing, the satellite terminal may determinetimeslot and burst frame timing, during acquisition, without having toderive it from the received signals. This can reduce the time needed foracquisition and thus reduce power. Terminals that are transmittingwithout receiving could also use the GNSS timing to transmit in specifictimeframes that could be pre-arranged. Additionally, the terminals coulduse the frequency reference as a basis for reducing the frequency errorin their transmission.

In some operating condition, channels may be shared with legacyservices, such as those associated with other mobile satelliteapplication like voice or packet data communication. This mode ofoperating is typically applied when the traffic volume is light (e.g.early in the deployment of a system or in beams that contain smallterminal population). In one embodiment of the invention, the sharedchannel can be a control channel in the legacy system, in which the dutycycle of forward link transmissions is approximately 25%-50%. The legacychannels can operate with Signal-to-Noise Ratios that are 25-30 dB abovethat needed by embodiments of the subject invention. For example, legacychannels can require Ec/No=0 dB to 5 dB, where Ec/No is the ratio ofenergy in a transmitted chip (or symbol for systems without spreading)to the noise power spectral density. As such, the same forward linkchannel can be used to simultaneously transmit the legacy signaling andthe spread signaling associated with embodiments of the subjectinvention. If needed, the power used for transmission of the legacysignals can be increased (typically by a small factor) to maintain theperformance of the legacy system. Reception, in the forward link, ofbursts associated with embodiments of the invention can be achieved ifthe level relative to the legacy transmissions provides sufficientSignal-to-Noise Ratio, (i.e. Ec/(Io+No) where Io is the interferencepower spectral density associated with the legacy waveforms. In thereturn link, the legacy system can use RACH transmission with low dutycycles. The transmissions associated with embodiments of the inventioncan also be transmitted with low duty cycle. Transmission associatedwith embodiments of the invention will typically have lower power thanthe legacy RACH transmission, so they can have a low impact on the RACHerror rate. As the legacy RACH transmission have low duty cycle,relatively short burst lengths, and power levels that don't necessarilyprevent reception, transmission associated with embodiment of theinvention can have reliable performance. As the performance of the linkscan be different during periods when the channel is shared with legacyservices, operation may be improved if the terminals are aware they aretransmitting on shared channels. The broadcast system information can beused to inform the terminals that they are operating on shared channels.

It will be recognized that while certain aspects of the invention aredescribed in terms of a specific sequence of steps of a method, thesedescriptions are only illustrative of the broader methods of theinvention, and may be modified as required by the particularapplication. Certain steps may be rendered unnecessary or optional undercertain circumstances. Additionally, certain steps or functionality maybe added to the disclosed embodiments, or the order of performance oftwo or more steps permuted. All such variations are considered to beencompassed within the invention disclosed and claimed herein.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the invention. Theforegoing description is of the best mode presently contemplated ofcarrying out the invention. This description is in no way meant to belimiting, but rather should be taken as illustrative of the generalprinciples of the invention. The scope of the invention should bedetermined with reference to the claims.

Although the invention is described above in terms of various exemplaryembodiments and implementations, it should be understood that thevarious features, aspects and functionality described in one or more ofthe individual embodiments are not limited in their applicability to theparticular embodiment with which they are described, but instead can beapplied, alone or in various combinations, to one or more of the otherembodiments of the invention, whether or not such embodiments aredescribed and whether or not such features are presented as being a partof a described embodiment. Thus, the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, should be construed as open ended as opposedto limiting. As examples of the foregoing: the term “including” shouldbe read as meaning “including, without limitation” or the like; the term“example” is used to provide exemplary instances of the item indiscussion, not an exhaustive or limiting list thereof; the terms “a” or“an” should be read as meaning “at least one,” “one or more” or thelike; and adjectives such as “conventional,” “traditional,” “normal,”“standard,” “known” and terms of similar meaning should not be construedas limiting the item described to a given time period or to an itemavailable as of a given time, but instead should be read to encompassconventional, traditional, normal, or standard technologies that may beavailable or known now or at any time in the future. Likewise, wherethis document refers to technologies that would be apparent or known toone of ordinary skill in the art, such technologies encompass thoseapparent or known to the skilled artisan now or at any time in thefuture.

The presence of broadening words and phrases such as “one or more,” “atleast,” “but not limited to” or other like phrases in some instancesshall not be read to mean that the narrower case is intended or requiredin instances where such broadening phrases may be absent. The use of theterm “module” does not imply that the components or functionalitydescribed or claimed as part of the module are all configured in acommon package. Indeed, any or all of the various components of amodule, whether control logic or other components, can be combined in asingle package or separately maintained and can further be distributedin multiple groupings or packages or across multiple locations.

Additionally, the various embodiments set forth herein are described interms of exemplary block diagrams, flow charts and other illustrations.As will become apparent to one of ordinary skill in the art afterreading this document, the illustrated embodiments and their variousalternatives can be implemented without confinement to the illustratedexamples. For example, block diagrams and their accompanying descriptionshould not be construed as mandating a particular architecture orconfiguration.

Moreover, various embodiments described herein are described in thegeneral context of method steps or processes, which may be implementedin one embodiment by a computer program product, embodied in acomputer-readable memory, including computer-executable instructions,such as program code, executed by computers in networked environments. Acomputer-readable memory may include removable and non-removable storagedevices including, but not limited to, Read Only Memory (ROM), RandomAccess Memory (RAM), compact discs (CDs), digital versatile discs (DVD),etc. Generally, program modules may include routines, programs, objects,components, data structures, etc. that perform particular tasks orimplement particular abstract data types. Computer-executableinstructions, associated data structures, and program modules representexamples of program code for executing steps of the methods disclosedherein. The particular sequence of such executable instructions orassociated data structures represents examples of corresponding acts forimplementing the functions described in such steps or processes. Variousembodiments may comprise a computer-readable medium including computerexecutable instructions which, when executed by a processor, cause anapparatus to perform the methods and processes described herein.

As used herein, the term module can describe a given unit offunctionality that can be performed in accordance with one or moreembodiments of the present invention. As used herein, a module might beimplemented utilizing any form of hardware, software, or a combinationthereof. For example, one or more processors, controllers, ASICs, PLAs,PALs, CPLDs, FPGAs, logical components, software routines or othermechanisms might be implemented to make up a module. In implementation,the various modules described herein might be implemented as discretemodules or the functions and features described can be shared in part orin total among one or more modules. In other words, as would be apparentto one of ordinary skill in the art after reading this description, thevarious features and functionality described herein may be implementedin any given application and can be implemented in one or more separateor shared modules in various combinations and permutations. Even thoughvarious features or elements of functionality may be individuallydescribed or claimed as separate modules, one of ordinary skill in theart will understand that these features and functionality can be sharedamong one or more common software and hardware elements, and suchdescription shall not require or imply that separate hardware orsoftware components are used to implement such features orfunctionality. Where components or modules of the invention areimplemented in whole or in part using software, in one embodiment, thesesoftware elements can be implemented to operate with a computing orprocessing module capable of carrying out the functionality describedwith respect thereto.

Furthermore, embodiments of the present invention may be implemented insoftware, hardware, application logic or a combination of software,hardware and application logic. The software, application logic and/orhardware may reside on a client device, a server or a network component.If desired, part of the software, application logic and/or hardware mayreside on a client device, part of the software, application logicand/or hardware may reside on a server, and part of the software,application logic and/or hardware may reside on a network component. Inan example embodiment, the application logic, software or an instructionset is maintained on any one of various conventional computer-readablemedia. In the context of this document, a “computer-readable medium” maybe any media or means that can contain, store, communicate, propagate ortransport the instructions for use by or in connection with aninstruction execution system, apparatus, or device, such as a computer.A computer-readable medium may comprise a computer-readable storagemedium that may be any media or means that can contain or store theinstructions for use by or in connection with an instruction executionsystem, apparatus, or device, such as a computer. In one embodiment, thecomputer-readable storage medium is a non-transitory storage medium.

What is claimed is:
 1. A communication system, comprising: at least oneterminal; at least one satellite in wireless communication with the atleast one terminal; at least one terrestrial hub in wirelesscommunication with the at least one terminal; and wherein the at leastone terminal uses the same communications architecture to communicatewith both the at least one satellite and the at least terrestrial hub.2. The communication system of claim 1, wherein the communicationsarchitecture includes an air interface.
 3. The communication system ofclaim 1, wherein the communications architecture includes an operatingfrequency.
 4. The communication system of claim 2 wherein the airinterface comprises Code Division Multiple Access (CDMA).
 5. Thecommunication system of claim 1, further comprising at least one networkinfrastructure in wireless communication with the at least one satelliteand the at least one terrestrial hub such that the at least one terminaland the at least one network infrastructure communicate with each otherthrough either the at least one satellite or the at least oneterrestrial hub.
 6. The communication system of claim 5, wherein the atleast one network infrastructure further comprises an informationelement which includes scheduled transmission information, wherein theat least one terminal is configured to communicate with the at least onenetwork infrastructure by sending a burst comprising a message at apre-scheduled time such that the at least one network infrastructure canderive a terminal identity for the at least one terminal by comparingthe time of the burst with the scheduled transmission information in theinformation element without having to include terminal identityinformation in the message.
 7. The communication system of claim 5,wherein the at least one network infrastructure further comprises atleast one satellite hub and a central server, wherein the at least onesatellite hub is in communication with the at least one satellite andthe central server is in communication with the at least one satellitehub such that the at least one terminal is in communication with thecentral server through either the at least one satellite and the atleast one satellite hub.
 8. The communication system of claim 6, whereintime-framing of the burst can be derived from waveforms applied inmodulation of a payload of the burst.
 9. The communication system ofclaim 6, wherein the pre-scheduled time is part of a pattern ofpre-scheduled times at which the at least one terminal sends burstscomprising messages to the at least one network infrastructure and thepre-scheduled times are a fixed period from each other.
 10. Thecommunication system of claim 5, further comprising at least onesatellite relay in wireless communication with the at least onesatellite and at least one terminal, wherein the at least one satelliterelay provides an alternative communication path between the at leastone satellite and the at least one terminal for situations in which theat least one terminal may be shadowed from the at least one satellite,wherein the at least one satellite relay is configured to receive burstsfrom the at least one terminal comprising messages at pre-scheduledtimes, apply a frequency offset to the bursts received from the at leastone terminal, and forward the bursts to the at least one networkinfrastructure through the at least one satellite and wherein the atleast one satellite relay is configured to receive messages from the atleast one network infrastructure through the at least one satellite,apply a frequency offset to the messages received from the at least onesatellite, and forward the messages to the at least one terminal.
 11. Acommunication system, comprising: network infrastructure including asatellite hub and a central server; a satellite in wirelesscommunication with the central server through the satellite hub; aterrestrial hub in communication with the central server; and a terminalin wireless communication with the terrestrial hub and the satellite;wherein, the terminal is configured to wirelessly communicate withcentral server through either the terrestrial hub or the satellite usingthe same communication architecture.
 12. The communication system ofclaim 11, wherein the communications architecture includes an airinterface.
 13. The communication system of claim 12, wherein the airinterface comprises Code Division Multiple Access (CDMA).
 14. Thecommunication system of claim 11, wherein central server furthercomprises an information element which includes scheduled transmissioninformation, where the terminal is configured to communicate with thecentral server by sending a burst comprising a message at apre-scheduled time such that the central server can derive a terminalidentity for the terminal by comparing the time of the burst with thescheduled transmission information in the information element withouthaving to include terminal identity information in the message.
 15. Thecommunication system of claim 14, wherein time-framing of the burst canbe derived from waveforms applied in modulation of a payload of theburst.
 16. The communication system of claim 14, wherein thepre-scheduled time is part of a pattern of pre-scheduled times at whichthe terminal sends bursts comprising messages to the central server andthe pre-scheduled times are a fixed period from each other.
 17. Thecommunication system of claim 11, further comprising a satellite relayin wireless communication with the satellite and the terminal, whereinthe satellite relay provides an alternative communication path betweenthe satellite and the terminal for situations in which the terminalcannot communicate directly with the satellite, wherein the satelliterelay is configured to receive bursts from the terminal comprisingmessages, apply a frequency offset to the bursts received from theterminal, and forward the bursts to the central server through thesatellite and wherein the satellite relay is configured to receivemessages from the central server through the satellite, apply afrequency offset to the messages received from the satellite, andforward the messages to the terminal.
 18. The communication system ofclaim 14, wherein, after receiving the burst, the central server isconfigured to determine the success or failure in reception of the burstand, in response, send an acknowledgement message to the terminal, theacknowledgement message consisting of either an ACK indicatingsuccessful reception of the burst or a NACK indicating failure ofreception of the burst, wherein energy required to transmit theacknowledgement message differs depending on whether the acknowledgementis an ACK or a NACK.
 19. The communication system of claim 18, wherein,if the acknowledgement message is an ACK, then the energy required totransmit the acknowledgement is negligible compared to if theacknowledgement message is a NACK.
 20. The communication system of claim19, wherein the energy to transmit an acknowledgement message is lessthan 50% if the acknowledgement message is an ACK compared to if theacknowledgement message is a NACK.
 21. The communication system of claim19, wherein the energy to transmit an acknowledgement message is lessthan 1% if the acknowledgement message is an ACK compared to if theacknowledgement message is a NACK.